1. Field of the Invention
The present invention relates to a radar device for detecting an object in front by radio waves, and more particularly to a radar device having two object detection sections.
2. Description of the Related Art
Radar devices installed on the front of a vehicle to measure distance to an object from the front of a vehicle and the relative velocity of the vehicle and the object have been proposed. By measuring the distance up to an object and the relative velocity, an alarm device for warning of an impending collision with an object in advance can be activated, or a brake can be activated to prevent collision with an object by using the measured distance and the relative velocity.
A phase mono-pulse type radar device is proposed as one of conventional radar devices. The phase-mono-pulse type radar device transmits a beam of an electromagnetic wave in a millimeter wave area, receives the wave reflected by an object using a plurality of receive antennas, and determines a direction of the object based on the phase difference of the receive waves.
FIG. 1 shows an example of receiving a reflected wave in the phase mono-pulse type radar device. Here θ is an angle to indicate the direction of an object when the front direction of the vehicle is 0°. The distance D is a space between two receive antennas, and the phase difference φ is a phase difference of the two receive waves. The two receive antennas 9a and 9b receive the waves reflected by the object existing at angle θ direction. The angle θ which indicates the direction of the object, which reflects the waves to be received by the two receiver antennas, is determined by the following expression.θ=sin−1(λφ/2πD)  [Expression 1]Here λ is a wavelength of the receive wave.
Also recently, as shown in Japanese patent Application Laid-Open No. H7-17342, a radar device, just like above, which further has a long range sensor and short range sensor so that the sensors are selectively used depending on the detection area to detect objects in a wide range, has been proposed. This radar device has a long range sensor for transmitting a radio wave which extends over a long range in a narrow range to detect a vehicle ahead that should be followed up, and a short range sensor for transmitting a radio wave which extends over a short range in a wide angle to detect peripheral objects which may be collided with.
In contrary, transmitting a radio wave which reaches a long distance in a wide angle and searching a detection range equivalent to that of a radar device having two sensors requires high output and is technically difficult, and also a large volume of information on unnecessary objects is detected, which increases the signal processing volume.
FIG. 2 is a conceptual diagram depicting a conventional radar device having a long range sensor and a short range sensor. The long range sensor 21 and the short range sensor 22 transmit radio waves to and detect objects in the respective object detection ranges. The long range sensor 21 and the short range sensor 22 supply the received signals to signal processing units 23 and 24 respectively. Then, based on the supplied signals, the signal processing units 23 and 24 determine the distance, direction and velocity of the objects. Information on the objects determined by the signal processing units 23 and 24 are supplied to an integration unit 25, and the integration unit 25 integrates the provided information. In particularly, information on an object detected by both sensors is integrated into single information.
In this radar device, in some cases a same object is detected by the long distance sensor and the short distance sensor. In such a case, information, including a distance to the detected object, are not always matched between the two sensors, depending on the measurement errors and the size of the detected object. Therefore using the detection result of an object by the two sensors, the radar device calculates the distance d to the object by the following expression.d=k·df+(1−k)·dn(k=0.9, 0.8, . . . , 0.1)  [Expression 2]
Here df is a distance to the object detected by the long range sensor, and dn is a distance to the object detected by the short range sensor. The value k decreased by 0.1 each time the object is detected. By performing this linear interpolation, the long range sensor is switched to the short range sensor.
The same is also performed for detection of the velocity of an object. Using the detection results of an object, the radar device calculates the velocity v of the object by the following expression.v=k·vf+(1−k)·vn(k=0.9, 0.8, . . . , 0.1)  [Expression 3]
Here vf is a velocity of the object determined by the long range sensor, and vn is a velocity of the object determined by the short range sensor. The value k decreases by 0.1 each time the object is detected. By performing this linear interpolation, the long range sensor is switched to the short range sensor.
However, an overlap of the detection areas generated in this radar device having a long range sensor and short range sensor requires analysis of the receive signals by both sensors. Since an object is detected using two sensors for a same range, and acquired signals are separately analyzed, a double operation volume is required to analyze the receive signals in the overlapped range. Analyzing the received signals from the two sensors in the overlapped range requires time and involves a waste of processing capabilities of the processor, which diminishes efficiency.
Furthermore, if it is performed an operation to determine which of the detection results by the two sensors is regarded as important, the processing capabilities of the processor even more wastes, which worsens efficiency.